Trunk muscle activity while lifting objects of expected and unexpected weight in normal subjects and patients with low back pain

BACKGROUND: Lifting heavy objects can induce postural stress and low back pain. OBJECTIVE: This study aimed to examine the effect of object weight expectations on trunk muscle activity and assess trunk muscle activity in people with chronic low back pain. METHODS: Twenty-two male college students (11 healthy and 11 participants with chronic low back pain) were recruited. The procedure was performed in three settings: lifting an expected 5-kg object, lifting an unexpected 10-kg object, and lifting an expected 10-kg object. Lifting was divided into five phases, and the muscle activity in each phase was compared between groups (chronic low back pain/control), object weight predictions, and phases. RESULTS: Compared to the control group, the chronic low back pain group had higher erector spinae muscle activity, regardless of weight or prediction, and significantly higher rectus femoris muscle activity in the early lifting phase of the expected 10 kg object (p= 0.043). Compared to when lifting the expected 10 kg object, erector spinae muscle activity was higher in the early lifting phase of the control group when lifting the unexpected 10-kg object (p= 0.016). CONCLUSIONS: Healthy individuals and individuals with chronic low back pain had different recruitment strategies for lifting objects heavier than predicted.